Field of the Invention
The invention relates to a method for determining a pulse sequence composed of multiple consecutive pulse sequence segments and a plurality of pre-pulses. The invention further relates to a method for operating a magnetic resonance installation. The invention also relates to a magnetic resonance installation that is designed to operate in accordance with such a method.
Description of the Prior Art
Within the MR scanner of a magnetic resonance installation, also known as a magnetic resonance tomography system, the body under examination is usually exposed to a relatively high basic magnetic field of e.g. 1, 3, 5 or 7 Tesla with the use of a basic field magnetic system. In addition, a magnetic field gradient is applied by a gradient system. High-frequency excitation signals (RF signals) are then transmitted via a radio-frequency transmit system by suitable antenna devices, in order to tip (deflect) the nuclear spin of specific atoms, which are resonantly excited by this radio-frequency field, by a defined flip angle relative to the magnetic field lines of the basic magnetic field. The relaxation of the nuclear spin is accompanied by an emission of radio-frequency signals, so-called magnetic resonance signals, which are received by suitable receive antennas and then processed further. The desired image data can then be reconstructed from the raw data acquired thus.
In order to perform a specific measurement (data acquisition), it is necessary to transmit a pulse sequence that include a radio-frequency pulse train and a gradient pulse train (with suitable gradient pulses in a slice selection direction, a phase coding direction and a readout direction, often in the x-direction, y-direction and z-direction of a Cartesian coordinate system) which must be switched (activated) in a coordinated manner. For the purpose of imaging, the timing within the sequence is particularly important, i.e. which pulses are consecutive and what time separations apply. A multiplicity of control parameter values is usually defined in a so-called measurement protocol, which is created in advance and can be retrieved for a specific measurement from a memory, for example, and can, if necessary, be modified locally by the operator, who can specify additional control parameter values such as e.g. a specific slice separation in a stack of slices to be measured, a slice thickness, etc. All of these control parameter values are then used as a basis for calculating a pulse sequence, which is also referred to as a measurement sequence, “MR sequence” (magnetic resonance sequence) or simply “sequence”. The pulse sequence is divided into pulse sequence segments, each of which has an excitation function and a readout function, these being assigned an excitation RF pulse and a readout signal, respectively, and having a time duration or period, also referred to as repetition time TR.
The readout functions for the magnetic resonance signals, i.e. the acquisition of raw data, are defined by data entry points in a memory organized as “k-space”, and this applies as well to the transmission of the radio-frequency signals. Any desired point in k-space can be accessed by switching the gradients in the various directions as appropriate. K-space is the spatial frequency domain, and a path of data entry in k-space (also referred to as a “k-space trajectory” or simply “trajectory” in the following) describes the route that is taken relative to time through k-space, when transmitting an RF pulse or performing a readout, as a result of corresponding switching of the gradient pulses. K-space is filled with raw data during a magnetic resonance measurement by scanning specific k-space trajectories during the raw data acquisition, and the image data is then reconstructed from this raw data by means of a Fourier transformation.
Various models can be used in order to fill k-space, e.g. Cartesian models in which individual sections of k-space trajectory are scanned on a line-by-line basis, for example, or possibly radial or spiral models. This depends inter alia on the respective sequence type.
Such pulse sequences are usually created by special sequence programmers. This creation is based on the precise definition and implementation of the individual gradient courses in this case, the precise timing and the shape and strength of the individual gradient pulses being specified by the sequence programmer according to the sequence type. Until now, the programming of the sequence has been closely linked to the hardware in this case, i.e. dependent on the respective type of magnetic resonance installation on which the MR sequence is to run.
MR sequences generally cause significant vibrations in the apparatus, and these vibrations are associated with loud noises. Depending on the type of scanner and protocol, noise levels considerably higher than 100 dB(A) can quickly be reached. The main cause of the noise is the rapid switching of the imaging gradients. Specifically, the magnetic fields of the imaging gradients are increased up to a maximal value within a very short time. Since the time specifications within a pulse sequence are generally very strict and the total duration of a pulse sequence, which determines the total duration of an MRT examination, must also be kept as short as possible, it is sometimes necessary to achieve gradient strengths of approximately 40 mT/m and slew rates of up to 200 mT/m/ms. In particular, such a pronounced steepness of the edge contributes to the known noise phenomena during the switching of the gradients. Eddy currents with other components of the magnetic resonance tomography apparatus, in particular the radio-frequency shield, are one reason for these nuisance noises. The rapidly changing magnetic field causes eddy currents in the structural elements of the apparatus, which can result in forces of attraction or repulsion. This results in minimal distortions and vibrations of the gradient coils, which are transferred to the overall system and are audible as loud noises.
Specific sequences such as the PETRA sequence (Grodzki et. al, MRM 2012) are characterized by extremely slow switching or slew rates of the gradient pulses. The resulting distortions and vibrations of the structural elements, e.g. the coils, are so small that they do not cause any noise to develop, and the measurement can be completely silent in principle. However, the obtained image contrast can be selectively modified further by means of so-called preparation pulses, also referred to as pre-pulses. These preparation pulses include e.g. preparation pulses for the suppression of fat and/or water, or T1 or T2 preparation pulses. Such preparation pulses are radiated into the examination object during the recording of the measured data, before or in combination with the RF excitation pulses that are used in the sequences. If e.g. a contrast is required in the PETRA sequence which necessitates the use of pre-pulses or preparation pulses, e.g. fat saturation pulses, the sequence is no longer silent. The reason for this is that in order to emit the pre-pulse, which usually occurs every n=5-25 repetitions, the gradients must be brought down, spoilers connected and the gradients brought up again. This means that gradients having a higher slew rate occur in the case of PETRA protocols with pre-pulses. This can be noticed in a very low-frequency rumbling/clanking of the scanner, such that the measurement is no longer silent. In the case of a repetition time of e.g. TR=4 ms and a number n=20 of repetitions after which a pre-pulse is generated, the frequency of the noises is e.g. 12.5 Hz. This results in a vibration and clanking of large components, since vibrations outweigh noises in this frequency range. Other sequences having limited gradient movement, to which the above cited problem applies, include, e.g., the silent sequences from the group of zero-TE sequences and the SWIFT sequence (SWIFT=Sweep Imaging with Fourier Transform).
This low-frequency vibration, which can be perceived by the patient as shaking, not only causes noise to develop, but can also result in the affected scanner components working loose or becoming detached.